Magnetic arc blowout device

ABSTRACT

Device for blowing out the arc struck between the contacts of electric switching devices, utilizing magnetic fields for the removal of the arc from the contact gap, and comprising, in combination with the magnetic means, at least one electric resistor, located outside of the extinction chamber proper and in such a manner as to be progressively inserted into the electric circuit that is connected with the contacts, due to the shifting of the related end of the arc length along the resistor(s), while the arc is being removed from the contact gap.

[56] References Cited UNITED STATES PATENTS 568,459 9/1896 Potter........................... 200/147 1,069,310 8/1913 317/78 FOREIGN PATENTS 991,926 9/1961 Great Britain................ ,200/144 1,008,660 ,9/1963 Great Britain................ 200/144' Primary Examiner-J. D. Miller Assistant Examiner-Ulysses Weldon Attorney-Me Glew and Toren ABSTRACT: Device for blowing out the arc struck between the contacts of electric switching devices, utilizing magnetic 317/11, fields for the removal of the are from the contact gap, and comprising, in combination with the magnetic means, at least 01h I18 one electric resistor, located outside of the extinction "02h 6 chamber proper and in such a manner as to be progressively inserted into the electric circuit that is connected with the 317/11; contacts, due to the shifting of the related end of the arc length along the resistor(s), while the arc is being removed 317/73 from the contact gap.

/ I I I6/ 1 Ernesto Maggi Ville Vittorio Emanuel 69, Ber [21] Appl. No. 758,307

Sept. 9, 1968 [45] Patented Apr. 20, 1971 Italy 20298A/67 200/147; 200/144, 144 (APRl); 317/74, 75, 78;

10 Ch, 12 Drawing Figs.

United States Patent [72] Inventor [221 Filed [32] Priority Sept. 9, 1967 [54] MAGNETIC ARC BLOWOUT bEVlCE [51] hit.

9 9 6 M J/ E I m5 3 7 r 1 I a F++++++k$i+++++ .ll l ill: a mlwfilw Rh 8 0 H J P wn, huh. w fiwwm a .11 1| '1 .l H R L mme. e 4 4 w l l 1 l l I 1 u| A 1 I I a g 5 I 3 9 M PATENTED APRZOIQYI sum 3 OF 4 INVENTOR. ERA/Es To me I hwam w ATTOHA/EYJ MAGNETIC ARC BLOWOUT usvrcs BACKGROUND AND SUMMARY OF THE INVENTION This invention relates to an arc blowout device, which has been carried into practice on the basis of a novel operating principle, that consists in having a high ohmic resistor gradually switched on in the circuit to be cut off by the continuous shifting action that is exerted on the electric are by magnetic fields, having suitably prearranged and dimensioned positions, polarities and intensities.

The ohmic value and geometric sized of this resistor, as well as the selection of a linear, or nonlinear characteristics therefor, are established in accordance with the features of the circuit to be cutoff, as e.g. nature of the current (DC or AC current), circuit voltage, estimated short circuit current of circuit, and residual current which can be extinguished by the extinction chamber forming part of the device in question, after the resistor has been switched on in the circuit.

The gradual switching on of the resistor in the circuit, as performed in the device in question, allows this resistor to be subdivided into a number of sections, whose ohmic values may be even different from one another, whereby to obtain the best cutting-off conditions for the related circuit, unlike what occurs in the conventional switch designs, fitted with resistors, and where these resistors are abruptly inserted into the circuit by expressly provided pairs of contacts or spark gaps, that allow the utilization of a resistor element having a preestablished single value for each pair of contacts or spark gaps.

The magnetic fields, by which the electric arc is acted upon in the device in question, can be provided either by permanent magnets, or by electromagnets whose coils are energized by the same current that flows through the circuit to be cut off. It is to be kept in mind that the intensity of permanent magnet fields is constant, while the fields of coils that are energized by the same current flowing through the circuit to be cutoff will have intensities changing in accordance with the current changes and that therefore are reduced proportionally with the reduction of this current, ensuing from the gradual insertion of the resistor into the circuit to be cut off. When an AC current is utilized, the sinusoidal course of current is followed also by the intensities of magnetic fields, which therefore become nil at each passage of current through the zero or neutral point.

The resistor, with which the device according to the invention is fitted, is independent from the shape of the extinction or quenching chamber proper by which the path and extension of the breaking are are delimited, and it may extend even to the outside of this chamber, without any dimensional restriction. The resistor may consist of elements made of metal alloys having a very high specific resistivity, as e.g. the Hadfields manganese alloys, as well as of coils whose wire. is preferably wound according to already known schemes, adapted to nullify the inductive effect of the winding, and which, owing to their relatively reduced sections, and to great lengths of resistance wire that can be wound thereon, allow to attain high ohmic values, even when made of conventional metals or alloys. When preferred, the resistors in question might consist also of bodies made of finely subdivided, electrically conductive materials, as e.g. carbon or metals, which particles are embodied in a ceramic material acting as binder. The bodies thus formed show very high specific resistivities, that may attain even a few thousand of ohms per cm.

As it can be readily appreciated, significant changes in the characteristics of the current and of the circuit to be cut off can be brought about by the above-described means.

In particular, by having high ohmic resistors series-inserted in AC circuits, the power factor thereof can be greatly improved, whereby, in the moment of the extinction of breaking arc, the peak value of recovery voltage that appears between the contacts, is remarkably reduced, thus allowing a real breaking of circuits with break gaps between the contacts smaller than those required in the absence of the resistors; thus, the voltage that really appears between the contacts of the subject device, when utilized for breaking AC circuits, is smaller than that which can be anticipated in the case of other types of extinction devices.

{The use of nonlinear resistors, consisting of bodies made of silicon carbide (carborundum) crystals, bonded by ceramic material, and having shapes similar to those of the abovementioned ceramic bodies, can be also practiced by the same means, and with arrangements similar to those adopted in the device in question, for these ceramic bodies.

A number of embodiment forms of the invention are shown, as nonrestrictive examples, in the accompanying drawings, to allow a better understanding of the inventive principle.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. I is a part sectional view of breaking device for an unidirectional or direct current circuit, and fitted with permanent magnets in order to provide magnetic fields adapted to shift the electric are, as well as with a resistor located at one end of arch extinction chamber, being the partial section taken on a plane perpendicular to the stroke of movable contact.

FIG. 2 is a sectional view of the same device, taken on a plane coincident with the stroke of the movable contact.

FIG. 3 is a plan view of the same device,

FIG. 4 is a sectional view taken on a plane coincident with the stroke of a movable contact, of a device, similar to that shown in FIG. 1, and having a resistor made up of two sections, fitted on opposite ends of the arc extinction chamber.

FIG. 5 is a sectional view, taken on a plane coincident with the stroke of a movable contact of a device, adapted for breaking uni- 0r bidirectional direct current circuits, or AC circuits, and fitted with two sections similar to those of FIG. 4, and located symmetrically opposite in respect of such stroke.

FIG. 6 is a sectional view, taken on a plane coincident with the stroke of a movable contact, of a device adapted for breaking unior bidirectional direct current circuits, or AC circuits, and fitted with an electromagnet energized by the same current to be cut off, as well as with a resistor that is located at one end of an arc extinction chamber.

FIG. 7 is a sectional view, taken on a plane coincident with the stroke of a movable contact, of a device adapted for breaking bidirectional direct current circuits, or also AC circuits, and fitted with permanent magnets, this device being a variant of the arrangement of contacts and resistors shown in FIG. 5.

FIG. 8 is a plan view of the device of FIG. 7, sectioned on a plane located above of the contacts.

FIG. 9 is a view of a device similar to that of FIG. 7, and having two consecutive arc extinction chambers utilizing common resistors.

FIG. 10 is a plan view of the device of FIG. 9, sectioned on a plane above the contacts.

FIG. I1 is a diagrammatic plan view of a particular arrangement of contacts, applicable to devices similar to those shown in the preceding FIGS.

FIG. 12 is a view of a breaking device similar to that of FIG. 2, but without the metal connections between the resistor and the contacts between which the arc is struck.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1, 2 and 3, the breaking device essentially consists of an arc extinction chamber, having sidewalls l of insulating material, and acting as a support of the chamber and of the following components:

a resistor, consisting of cylindrical ceramic bodies 2, and of plates 3, made of an electrically conductive material, a good electric contact between the flat ends of the bodies being obtained by the pressure that is exerted thereon by the clamping screw 4,

the rear born 5, made of a material having a good electrical conductivity, and fonned with an extension that is electrically connected, by the flexible conductor 6 and the spring-biased contact roller 7, with the movable contact I4,

the pair of permanent magnets 8, fitted on that side of ceramic bodies 2 directed toward the inside of the chamber. and provided with the return circuit 9, made of a magnetic material, for the magnetic flux provided by magnets 8 outside of the chamber.

the pair of permanent magnets 10, located on opposite sides of stationary contact 11 of the breaking device, and provided with the return circuit I2, made of a magnetic material, for the magnetic flux provided by magnets I outside of the chamber,

the pairs of permanent magnets 13 fitted along the walls 1 above movable contact I4, as well as along the travel of the electric arc, and all provided with return circuits, made of a paramagnetic material, and consisting of the supports I5 for magnets I3, of yokcs I6 fitted outside of the chamber and of yokcs l7 fitted inside of the chamber; the magnetic systems consisting of magnet pairs I3 and of related return circuits being subdivided into sections that are separated from one another, whereby to ensure the insulation of extinction chamber in the longitudinal direction, parallel to the travel of movable contact 14,

the pair of permanent magnets 18 fitted on opposite sides of rear horn 5 and provided with the return circuit 19, made of a paramagnetic material, for the magnetic flux provided by magnets 8, outside of the chamber,

the protective shield 20, made of an insulating material, resisting the direct action of the electric are, for the pairs 8 and 10 of permanent magnets,

the protective shields 22 made of an insulating material, resisting the direct action of the electric are, for the yokes 17,

the protective shields 23 made of an insulating material, resisting the direct action of the electric arc, for the pair 18 permanent magnets, and

the electric connection 24 between the electrically conductive plate 25, by which the resistor is supported, and the fixed contact II.

The arrangement of extinction chamber walls I, and of all the other above-itemized components, is such as to form a longitudinal slit 26, lying in the middle vertical plane taken through the stroke of movable contact 14, and which represents the extinction for quenching chamber proper, wherein the electric arc is struck, slit 26 extending, tapered upwardly, up to the crossing point of protective shields 22 of yokcs 17, and forming a labyrinth path for the electric are that comes from below due to the action of the magnetic fields of the blowout device, defined by the inclined lower portions 27 of yokcs l7 and by the alternate arrangement thereof.

The absolute dimensional independence of the resistor and its components 2, 3 and 25 from the width of slit 26 is clearly shown in the FIGS. 2 and 3, slit 26, as already stated, forming the proper extinction chamber by which the path and extension of are are delimited.

The fields which are brought forth inside of the extinction chamber by the permanent magnet pairs 8, l0, l3 and 18 are all directed in the same sense, as conventionally indicated by little crosses on the drawings, i.e. they are directed to enter in the plane of drawing; the direct current of circuit to be cut out is directed from the fixed contact 11 toward the movable contact 14.

When the breaking device is opened, the movable contact 14 is drawn away from the fixed contact 11, running through the lower portion of slit 26 in the direction as indicated by the arrow in FIGS. 2 and 3, under the action of a control (not shown) of an already known type; the are that is struck between the drawn-away contacts is at once subjected to the action of a crosswise directed magnetic field, as provided by the pair of magnets 10, whereby it is urged toward the upper zone of slit 26, as indicated by way of example by reference numeral 28 in the FIG. 2.

The action exerted on the are by the magnetic fields of magnet pairs 8 and 13 is a continuous one, and works on the whole length of the are thus urging it toward the top of slit 26, and causing its root to leave the front side of fixed contact II, contacting firstly the conductive plate 25 and then, in succession, the ceramic bodies 2 and the conductive plates 3, whereby the whole resistor is gradually inserted into the circuit to be cut out.

Moreover, the action exerted by the magnetic fields of magnet pairs 13 on the are, which is no more parallel to the travel of movable contact 14, is such that the opposite arc end is urged to travel across the upper edge of movable contact 14, in the direction of motion thereof, thereby taking positions like that indicated, by way of example, by the reference numeral 29 in FIG. 2, and until attaining the horn 5 by which the electric continuity with the movable contact is maintained through the flexible conductor 8 and the roller contact 7. His arc end is freely enabled to obey even the action tending to shift the arc upwardly, as exerted by the magnetic fields of magnet pair I3, and will leave the movable contact I4, to climb the horn 5, as indicated by way of example by the reference numeral 30 in FIG. 2, coming thus under the action of the magnetic field of magnet pair I8, and by which it is urged to climb further on the horn, toward the tip thereof.

The electric arc is thus quickly removed from the gap between the separating contacts, to enable the insulating properties of the medium that is present therebetween to be restored as quickly as possible. In the meantime, the resistor, by which the current to be cut off is limited, has been gradually inserted into the related circuit, thereby reducing the current down to values such as to allow the final extinction of the are, due to the action of the deionizing and insulating medium wherein the arc is stretched more and more, inside of the slit 26 and of labyrinth path as defined by the insulating walls 221- After the extinction of the arc and the cutting out of the current, the recovery voltage that appears between the contacts, whose gap has been in the meantime increased to a preestablished value, is no longer sufficient to restrike the arc, puncturing the interposed insulating medium (air or other insulating fluid), whereby the circuit is definitely broken.

In some cases, it will be possible to dispense with the use of shields 22 by which the yokcs I7 are protected; then, the metal surfaces of these yokcs are enabled to assist the arc cooling action, thus facilitating the deionization and the final extinction of the arc.

In some other cases, it may be convenient to strenghten the dragging action, exerted on the arc inside of the labyrinth defined by the yokcs l7 and by the insulating coverings 22 thereof, by the upward shifting of both arc ends, driven by the magnetic fields of the magnet pairs 8 and 18. Such strcnghtening can be obtained by recourse to additional pairs of permanent magnets, fitted on the oppositely facing walls of yokes 17, whereby to the form suitable magnetic fields, directed crosswise to path of the are, similarly to what disclosed in the specification of another patent, in the name of same applicant.

At any rate, the presence of crosswise yokcs 17 serves also to prevent the escape, and the free development, of the electric are out of the extinction chamber.

FIG. 4 shows the same components of FIGS. 1, 2 and 3, except for the rear horn 35, which is shortened and shaped in such a manner as to act as a support for a resistor that is located at the rear end of the extinction chamber, and which consists of ceramic bodies and contact conductive plates, similar to those of the preceding example, indicated by the reference numerals 32' and 33' respectively, and which also constitute the resistor located at the fore-end of same chamber. Such arrangement allows to obtain a better gradualness of the insertion of resistance in the electric circuit to be interrupted.

The directions of the magnetic fields provided inside of the extinction chamber by the permanent magnet pairs 38, 40, 43 and 48, are conventionally indicated by little crosses in FIG. 4.

When the breaking device is opened, the movable contact 44 is drawn away from the fixed contact 41, in the direction of the arrow as shown in FIG. 4, under the action of 'a switching control of an already known type (not shown). The electric arc struck in the contact gap is subjected at once to the action of the crosswise magnetic field of the pair of permanent magnets 48, and is thus urged toward the upper zone of the vertical slit between the magnet pairs 40, 38 and 43, which represents the extinction or quenching chamber proper, wherein the electric arc extends, taking a position similar to that indicated, as an example, by the reference numeral 58 in MG. 4.

The action that is exerted on the are by the magnetic fields of the magnet pairs 38 and 43 is a continuous one, and works on the whole length of, thus urging it toward the top of the slit and causing its root to leave the front side of fixed contact 41 contacting firstly the conductive plate 55 and then, in succession, the ceramic bodies 32 and the conductive plates 33, whereby the whole resistor, located toward the fore-end of the chamber, is gradually inserted into the circuit to be cut off.

The action exerted by the magnetic fields of magnet pairs 43 on the are, which is no longer parallel to the travel of movable contact 44, is such as to urge the opposite root of the arc to travel across the upper edge of movable contact 44, as shown, by way of example, by the reference numeral 59 in FIG. 4, until attaining the horn 35, whose electric continuity with movable contact 44 is maintained through the flexible conductor 36 and the roller contact 37. This are root is therefore enabled to obey even the action tending to shift the arc upwardly, as exerted by the magnetic fields of magnet paris 43, and will leave the movable contact 44, to climb the born 35, as well as the resistor consisting of bodies 32' and of plates 33', located toward the rear end of the extinction chamber, as indicated by way of example by the reference numeral 60 in the FIG. 4, coming thus under the action of the magnetic field of the magnet pair 48, and by which it is urged to climb further on toward the upper end of the rear resistor.

The electric arc is thus quickly removed from the contact gap, thus enabling the insulating properties of medium that is present therein to be restored as quickly as possible. At the same time, the larger part of the resistor by which the current is limited, has been gradually inserted, by the action of the arc, into the circuit, thereby reducing the arc current down to values such that the arc is made unstable, and can be definitively extinguished. due to the action of the insulating and deionizing medium, in which the arc is being stretched more and more, inside of the slit and the labyrinth path as defined by the insulating walls 52.

Also in the better case, like the preceding one, the recovery voltage that appears in the contact gap is no longer sufficient to restrike the arc, whereby the circuit is definitively cut off. Even in the considered case, it is possible to dispense with the use of shields 52, by which the yokes 47 are protected, or pairs of permanent magnets may be added for the same purposes already mentioned with reference to FIGS. I, 2 and 3 FIG. 5 shows a breaking device consisting of two sections, each similar to the extinction chamber as shown in the FIGS. 1, 2 and 3, said sections being located symmetrically opposite to a middle plane that contains the straight line traveled by the movable contact of the device, in order to allow the interruption of electric currents directed both from the fixed contact 71 toward the movable contact 74, and from the movable contact 74 toward the fixed contact 71.

The last-mentioned case, i.e. the breaking of a DC circuit, wherein the switching device is inserted in such a manner that the current will flow from the movable contact 74 toward the fixed contact 71, is considered by way of example in the FIG. 5.

When the breaking device is opened, the movable contact 74 is drawn away from the fixed contact 71 in the direction of the arrow of FIG. 5, under the action of a control device (not shown) of an already known type. The are that struck in the contact gap is subjected to the action of crosswise directed magnetic fields of the permanent magnet pairs 68, 70, 73and 78, corresponding to the previously described ones, thereby taking in succession the positions as indicated, by way. of example, by the numerals 88, 89 and in the FIG. 5, until attaining the lower end of horn 65 of the bottom section of the chamber. The are is thus quickly removed from the contact gap, in order to enable the medium that is present therein to recover as quickly as possible its insulating properties. At the same time, the resistor by which the current to be cut off is limited, has been gradually inserted into the circuit, thereby reducing the arc current to values such that the arc is made unstable, and can be definitively extinguished, due to the action of the deionizing andinsulating medium, wherein the arc is more and more stretched, inside of the central slit and labyrinth path as defined by the insulating walls 82. The conditions resulting from the extinction of the are are such as to positively prevent a restriking of arc in the gap between the contacts.

Even in the device as shown in FIG. 5, it is possible to abstain from the use of shields 82 by which the yokes 77 are protected, or pairs of permanent magnets may be added for the same purposes already mentioned with reference to the FIGS. 1, 2 and 3.

As it can be readily appreciated, the device as shown in FIG. 5 is also suitable for cutting off AC circuits, when considering that an alternating current consists of a succession of halfwaves that are alternately directed from the fixed contact 71 toward the movable contact 74, and vice-versa. On the basis of such consideration, it will be obvious how the device of FIG. 5 can be utilized for the breaking of an AC circuit wherein, in the moment of separation of contacts, the current half-wave directed from the movable contact 74 toward the fixed contact 71 is present; the shifting of arc will then occur in the direction as indicated by way of example, by the successive positions 88, 89 and 90 of arc, while the extinction thereof will usually take place when the zero point is crossed by the wave.

Obviously, when the oppositely directed current half-wave- -i.e. that directed from the fixed contact 71 toward the movable contact 74-is instead present in the circuit, then the arch will be shifted toward the upper section of the extinction chamber, thus taking successively positions similar to those as indicated, by way of example, by the reference numerals 28,

29 and 30 in the FIG. 2.

The very same case may occur for the restriking of the are between the contacts which would have not attained, at the first crossing of the zero point by the alternating current, as present in the initial arc with a half-wave directed from the movable contact 74 toward the fixed contact 71, a gap sufficient to allow the medium present therein to resist the recovery voltage which appears between the contacts; then, the current of a new are will have the form of a half-wave extending in a direction opposite to the preceding one, whereby the arc is shifted toward the inside of the chamber section that has not yet been affected by the interruption. This time, the duration of the arc is equal to that of a whole halfwave, i.e. of 10 milliseconds for a circuit frequency of 50 Hz, and thus such as to allow to attain, at the next crossing of the neutral line, a gap width sufficient to enable the fluid that is present in the gap to successfully withstand the recovery voltage that appears between the contacts, whereby the circuit will be definitively broken.

FIG. 6 shows the longitudinal section of a breaking device that consists of an extinction chamber, utilizing the magnetic field provided by a coil 99, made of an electrically good conductor wire, wound on a paramagnetic core 98, and insulated therefrom, whereby to attain an are shifting inside of the chamber, such as to cause the insertion of a high ohmic resistance into the circuit to be cut off.

The extinction chamber is laterally delimited by the walls 91, made of an insulating material, and acting as a support for all other components, which are wholly similar to those of the previously described devices. In the case in question, the resistance is subdivided into two sections, consisting of the cylindric ceramic bodies 92 and 92', and of conductive plates 93, that are tightly clamped with another by the locking bolts and 94.

The magnetic field of the blowing-out electromagnet, extending in the direction as conventionally indicated in the drawing by small crosses, corresponds to the case in which the direct current of the circuit to be cut off, and flows also through the coil 99, is applied to terminal 103 and passes from the fixed contact 101 to movable contact 104. The same representation holds also in the case of an alternating current, when it is considered for the period of time in which the corresponding half-wave extends in the same direction as the direct current.

When the breaking device is opened, the movable contact 104 is drawn away from the fixed contact 101, traversing the lower section of the chamber, between the walls 91, in the direction as shown by the arrow of FIG. 6, under the action of a control mechanism of an already known type. The electric arc struck in the gas between the drawn-away contacts, is subjected at once to the action of the crosswise directed magnetic field of the blow out electromagnet, and is urged toward the upper section of the chamber, as shown by way of example by the reference numeral 108 in FIG. 6.

The action exerted on the are by the magnetic field is a continuous one, and works on the whole length thereof, thus urging it toward the chamber top, and causing its root to leave the front side of fixed contact 101, contacting firstly the conductive plate 102 and then in succession the ceramic bodies 92 and the conductive plates 93, thereby gradually inserting one section of the resistor into the circuit to be cut off.

The action that is exerted by the magnetic field on the are, which later is no longer parallel to travel of movable contact 104, is also such as to urge the opposite root thereof to travel across the upper edge of movable contact 104, and in the same direction thereof, thus taking positions similar to that indicated, by way of example, by the referencenumeral 109 in the HO. 6, until attaining the horn 95 by which the electric continuity with the movable contact 104 is maintained through the flexible conductor 96 and the roller contact 97. This are root is therefore enabled to obey the action tending to shift the arc upwardly, as exerted by the blowing-out magnetic field, and will leave the movable contact 1114, to climb firstly the horn 95, and then the second section of the current limiting resistor --as shown by way of example by the line 110 in the FIG. 6 -the latter resistor being formed by the bodies 92' and 93.

The electric arc is thus quickly removed from the contact gap, and consequently also quickly blown out, as described in the preceding cases.

In the case of an alternating current, the arc will usually be blown out at the first crossing of the neutral line by its current.

FIGS. 7 and 8 show a breaking device similar to that of FIG. 5, Le. consisting of 1 two extinction devices located symmetrically opposite with respect to a plane extending along the path of the movable contact, with the exception that each one of these devices is made up by two consecutively arranged arc extinction chambers, by which the resistors, consisting of bodies 153 and of plates 138 are utilized in common. This is because the contact system consists of three contacts, i.e. the fixed contact 121, the usual movable contact 124, and an intermediate movable contact 1311. 111 indicate the insulating walls by which all other components are supported. The particular purpose of such arrangement is mainly to facilitate the interruption of high current intensities, having relatively high voltages, by subdividing the are into two sections, inside of two extinction chambers, that are consecutively located, but separated from each other, whereby the ionization effect, as caused by the currents that are successively flowing therethrough, is cut down in the second chamber, owing to the insertion of a resistor that is brought about by the initial arc stnick in the first chamber.

The operation of the breaking device shown in the FIGS. 7 and 8 will be explained in more detail in the following description.

This device essentially consists of two consecutively arranged arc extinction chambers, having sidewalls 111 made of an insulating material, and that act also as a support for the chamber, and for the hereinafter itemized components:

Two resistors consisting of bodies 112 and of conductive plates 113, that are tightly clamped together by the locking bolts 114.

Two resistors consisting of the ceramic bodies 153, along with the conductive plates 138 and the locking bolts 148. The sizes of bodies 153 are smaller than those of bodies 112, because they are traversed by already reduced currents, owing to the insertion of bodies 112 into the circuit.

The support 131, made of a highly conductive material and designed to support the two resistors, and the intermediate contact 130, having the stop extensions 136, and connected with the support 131 by the flexible conductors 134.

The rear horns 115, also made of a good electrically conductive material, and formed with extensions that are electrically connected, by the flexible conductors 116 and the spring contact rollers 117 with the movable contact 124.

The permanent magnet pairs 118, fitted along the front side of ceramic bodies 112 directed toward the chamber inside, and that are provided with the related return circuits 112, made of a paramagnetic material.

The permanent magnet pairs 120, fitted on opposite sides of fixed contact 121 of breaking the device, and provided with the related retumcircuits 122, made of a paramagnetic material, for the corresponding magnetic fields, outside of first chamber.

The permanent magnet pairs 123, fitted along the walls 111 of the first chamber, both above and below the intermediate contact 130, and extending along the travel of the electric arc portion struck in the first chamber, are provided with return circuits of paramagnetic material. These return circuits consist of plates 149, supporting the permanent magnets,

outer yokes 126 and yokes 127 located inside the first chamber. The magnetic systems including the permanent magnets and the associated return circuits are subdivided into separate sections to ensure insulation of the first extinction or quenching chamber in a direction along and parallel to the path of travel of intermediate contact 130.

The pairs of permanent magnets 133, fitted along the wall 111 of the second chamber above, below and on either side of movable contact 124, as well as across the path of the portion of are that struck in the second chamber, and which are provided with return circuits, consisting of the magnet supports 150, of yokes 147 on the outside of the second chamber and of yokes 139 inside of the second chamber. The magnetic systems, consisting of magnet pairs 133 and of related return circuits, are subdivided into sections separated from one another, whereby to ensure the insulation of the second extinction chamber in a direction longitudinal and parallel to the path of movable contact 124.

The permanent magnet pairs 128, located on either sides of rear horns 115, and provided with the return circuits 129, made of paramagnetic material, for the corresponding magnetic fluxes as brought forth by them, outside of second chamber.

The shields 154 and 155, made of an insulating material resisting the direct action of the electric arc, for the pennanent magnets of pairs 118 and 120, respectively, as well as similar protective shields for the permanent magnets of pairs 123 and 133.

The shields 132 and 140, made of an insulating material resisting the direct action of the electric are, for the yokes 127 and 139, respectively.

The electric connections 134 between the conductive plates 135, by which the resistors of the first chamber are carried, and the fixed contact 121.

The arrangement of walls 111 of the first chamber, as well as of the above-itemized components, is such as to form two identical longitudinal slits 151, lying in the vertical plane wherein the travel of intermediate contact 130 also lies, and which represent the extinction or quenching chamber proper wherein the electric arc is struck. These slits extend, while narrowing, in the direction of protective shields 132 of yokes 127 which, due to the inclination of parts directed to the inside of same chamber, and the alternate arrangement of the inclined parts, form a labyrinth path for the electric are that comes from the inside of the first chamber, due to the action of the magnetic fields of blowout device.

Two identical slits 152, similar to the preceding ones, are

, formed in the second chamber by the components fitted therein and supported by the walls 111; slits 152 extend, while narrowing, in the direction of protective shields 140 of yokes 139 which, due to the inclination of parts directed to the inside of the chamber, and to the alternate arrangement of these inclined parts, form a labyrinth path for the electric are that comes from the inside of the second chamber, due to the action of the magnetic fields of the blowout device.

The magnetic fields, inside of the first chambers of the permanent magnet pairs 118, 120 and 123, acting therein, are all turned in the same direction, as conventionally indicated by little crosses in the drawing, i.e. directed in the plane of drawing.

The magnetic fields, inside of the second chamber, of the permanent magnet pairs 133 and 128, acting therein, are all turned in the same direction, which is, however, opposite to that of fields of the first chamber, as conventionally indicated by dots on the drawings; otherwise stated, they are directed out of the plane of drawing.

The direct current, or the half-wave of alternating current, flowing through the circuit to be cut off, and that is present at the very moment of initial contact separation, is assumed as directed from the fixed contact 121, toward the intermediate contact 131) and the movable contact 124.

When the breaking device is closed, the intennediate contact 130 is tightly clamped against the fixed contact 121, under the thrust of movable contact 124 and overcoming the compression resistance of springs 137, which are forced by the extensions 136 against the forewall of support 131, the movable contact 124 being operated by any type of control means for electric switching devices (not shown).

When the above device is being opened, the movable contact 124 goes back along with the intermediate contact 130, which is acted upon by the springs 13'], running along a straight line parallel to slits 151 and 152, in the direction of arrows of FlGS. 7 and 8. A first electric arc is thereby struck in the first extinction chamber, and namely in the gap between the fixed contact 121 and the intermediate contact 130, this are being subjected at once to the action of the crosswise directed magnetic field of the magnet pair 120, and thus urged to move toward the upper section of slit 151, as shown by way of example by the reference numeral 141 in FIG. 7.

As previously stated, a continuous action is exerted on the arc length by the magnetic fields that are brought by the magnet pairs 118 and 123, whereby the arc is quickly shifted upwardly, thus leaving the front side of fixed contact 121, and contacting in succession the conductive plate 135, the ceramic bodies 112 and the conductive plates 113, and consequently inserting a part of the resistor into the circuit to be cutoff.

The intermediate contact 130 has, in the meantime, completed its stroke, and is stopped when its extensions 136 abut against the rear part of support 131; the movable contact 124 is thus drawn away from the intermediate contact 130, and an arc length is struck in the gap thus formed, inside of the second extinction chamber, this are being subjected at once to the action of the crosswise directed magnetic fields of the permanent magnet pairs 133.

By assuming that the direction of circuit current has not been reversed in the meantime, then the arc length in the second chamber will be urged to move downwardly by the action of the above fields, thus taking the position as indicated by way of example by the numeral 144 in the FIG. 7. The intensity of the related current is however reduced with respect of the current intensity of the arc struck in the first extinction chamber, owing to the insertion into the circuit of a part of the resistors consisting of the bodies 112, as caused by the arc length in the first chamber. Thus, the ionization of the medium that is present in the gap across which the arc is struck in the second extinction chamber, will be much lower.

The are lengths will be shifted further on, in the two consecutive chambers, similarly to what has been previously stated with reference to the other embodiments of the inventive idea, thus taking the positions as indicated, by way of example, by the reference numerals 142 and 143 in the first chamber, and 145, 146 in the second chamber (see FIG. 7), and thereby inserting one of the resistors consisting of bodies 112, as well as both resistances consisting of bodies 153, into the circuit to be cut oif.

Thus, the electric arc is quickly withdrawn from the contact gaps, in order to allow the medium present therein to recover as quickly as possible its insulating properties.

The intensity of current to be cut off will be reduced to values such that a marked instability is imparted to the arc, which is then extinguished much more easily by the action of the deionizing and insulating medium in the inside of slits 151 and 152, as well as inside of the labyrinth paths formed by the walls 132 and 140.

In the case of alternating current, the extinction of the electric are usually occurs at the first crossing point, through the neutral line of same current. However, should the gap between the contacts of the breaking device not have attained at that moment a width sufficient to allow the medium present therein to withstand the recovery voltage that appears across the contacts, then an arc would be restruck therethrough, and a current will flow, having the form of a half-wave, in a direction opposite to the preceding one. Then, the action of the magnetic fields on both are lengths restruck in the two chambers will be similar to that as previously described, except that it is exerted simultaneously in both chambers, and will cause shiftings of the arc lengths toward chamber sections opposite to those as previously stated, in FIG. 7. The duration of the new arc lengths are, this time, equal to that of a whole half-wave, i.e. of l0 milliseconds for a current having a frequency of 50 Hz, and thus such as to allow the movable contact to attain, at the next crossing of the zero line, a spacing from the intermediate contact sufficient to enable the medium present in the gap, to successfully withstand the recovery voltage that appears across the contacts of the breaking device.

FlGS. 9 and 10 show a variant of the breaking device that of FIGS. 7 and 8, and wherein a single resistor, interposed between the two consecutive extinction chambers, is substituted for the resistor pairs as previously utilized, which results in a structural simplification of the device, and consequently also a substantial reduction of the cost thereof.

This variant essentially consists of two consecutively located chambers for the extinction of the electric arc, and having the sidewalls 161, made of an insulating material, and which act as a support for chambers, and for the hereinafter itemized components:

the single pair of resistors, consisting of cylindric ceramic bodies 162 and of conductive plates 163, by which a good electric contact between the flat ends of bodies is ensured by means of the pressure that is exerted thereon by the locking bolts 16 1;

the forehoms 165, made of a good electrically conducting material and formed with extensions that are electrically connected with the fixed contact 167 by means of flexible conductors 166;

the rear horns 168, also made of a good electrically conducting material, and formed with extensions that are electrically connected with the movable contact 171 by the flexible conductors 169 and the spring-biased rollers 170;

the support 172, made of a good electrically conducting material, and by which the resistors and the intermediate contact 173 are supported, the intermediate contact being formed with the stop extensions 174, and electrically connected with support 172 by the flexible conductor 175;

the permanent magnet pairs 177, located on opposite sides of forehoms 165 and provided with the return circuits 178, made of a paramagnetic material, for the magnetic fluxes which are brought outwardly of the first chamber;

the permanent magnet pairs 179, located on opposite sides of fixed contact 167 of the breaking device, and provided with the related return circuits 180, made of a paramagnetic material, for the magnetic fluxes brought outwardly of the first chamber;

. the permanent magnet pairs 181, located on opposite sides of rear horns 168, and provided with the related return circuits 182, made of paramagnetic material, for the magnetic fluxes brought outwardly of the second chamber;

the permanent magnet pairs 183, located along the wall 161 of the first chamber, both above and below the intermediate contact 173, along the travel of the arc length that is struck in the first chamber, and provided with return circuits, made of a paramagnetic material, for the magnetic fluxes brought forth by them outwardly of the first chamber; these latter circuits consisting of the plates 184, by which the magnets are supported, by the yokes 185 fitted outwardly of the first chamber, and by the yokes 186 fitted inside of this chamber. The magnetic systems, consisting of magnet pairs 183 and of related return circuits, are subdivided into sections separated from one another, whereby to insure the insulation of the first extinction chamber in a direction longitudinal and parallel to the travel of intermediate contact 173;

the permanent magnet pairs 188, located along the walls 161 of the second chamber, above, below and on opposite sides of movable contact 171, across the travel of the electric arc length struck in the second chamber, and that are provided with return circuits made of a paramagnetic material and consisting of the supports 189, by which said magnets are supported, of the yokes 190 located on the outside of the second chamber, and of yokes 191 located inside thereof, the magnetic systems, consisting of pairs 188 and of related retum circuits subdivided into sections separated from one another, whereby to ensure the insulation of the second extinction chamber in a longitudinal direction, parallel to the travel of movable contact 171;

the shields 193 and 194, made of an insulating material resisting direct action of the electric arc, and by which the permanent magnets of pairs 177, 179, 181, 183 and 188 are protected;

' the shields 187 and 192, made of an insulating material resisting direct action of the electric arc, and by which the yokes 186 and 191 are respectively protected;

the electric connections 166 between the fore horns 165 and the fixed contact 167.

The arrangement of walls 161 of the first chamber, and of the above-itemized components, is such as to form two identical slits 195, longitudinally extending in the same middle vertical plane, wherein the travel of intermediate contact 173 also lies. Such slits, which represent the extinction or quenching chamber proper, wherein the electric arc extends, are protracted, while narrowing, in the direction of protective shields 187 of yokes 186 by which, due to the inclination of their parts directed toward the inside of the chamber, and to the alternate arrangement of the inclined parts, a labyrinth path is defined for the electric are coming from the inside of the first chamber, owing to the action of the magnetic fields of the blowout device.

Two slits 196, identical with each other and similar to the previously described slits, are formed in the second chamber by the components that are fitted therein. Such slits extend, while narrowing, in the direction of protective shields 192 of yokes 191 by which, due to the conformation of their parts directed toward the inside of the chamber, and the alternate arrangement of these inclined parts, a labyrinth path is defined for the electric are coming from the inside of the second chamber, owing to the action of the magnetic fields of the breaking device. The magnetic fields inside of the first chamber, of the permanent magnet pairs 177, 179 and 183 acting therein, are all directed in the same direction, corresponding to that which is conventionally indicated by little crosses on the drawing; otherwise stated, they are directed toward the plane of the drawing.

The magnetic fields inside of the second chamber, of the permanent magnet pairs 181 and 183 acting therein, are all directed in the same direction, which is, however, opposite to that .of the fields in the first chamber; otherwise stated, they go away from the plane of the drawing.

The direct current, or the half-wave of alternating current that is present at the moment the contacts start to move away from each other, and which flows through the circuit to be cut off, is directed from the fixed contact 167, toward the intermediate contact 173 and the movable contact 171. The behavior of different components of the device as shown in FIGS. 9 and 10 is wholly similar to that already described with reference to FIG. 7 so that the description is not repeated herein.

The embodiment of the invention as shown in the FIG. 12, is wholly similar to that shown in FIGS. 1, 2 and 3, except for the absence of metal connections between the contacts and the resistors 2-3, and the horn 5 respectively. In more detail, the flexible connection (indicated by the reference numeral 24 in FIG. 2) is not provided in the extinction chamber of FIG. 12, between the fixed contact 11 and the bottom plate 25 of the resistor, consisting of bodies 2 and 3, and the flexible connection is not provided between the contact 14, the horn 5 and the roller running on the movable contact 14 (respectively indicated by the reference numerals 6 and 7 in the FIG. 2).

The above connections, as well as the similar ones that are provided in the embodiments shown in FIGS. 4 to 10 inclusive, have proved themselves, since they can be all substituted by the electric are, when struck between the contacts (11 and 14 in the FIG. 12) at the very start of opening thereof, and which is then shifted toward the top of slit 26 under the action of the magnetic fields in the extinction chamber, of the magnet pairs 8 and 13, whereby the arc root, initially lying on the fixed contact 11, will firstly contact the plate 25, thus establishing a connection bridgediagrammatically shown in FIG. 12 and indicated by the reference numeral l2-with the fixed contact 1 l, and then the ceramic bodies 2 and the plates 3, whereby the whole resistor 2-3 is gradually and automatically inserted into the circuit to be cut off. Something something similar to what has been described above also occurs at the opposite arc end, i.e. even the other are root is shifted, under the action of the abovestated magnetic fields, across the upper edge of movable contact 14, in the direction of travel thereof, thus taking positions similar to that indicated by the reference numeral 29 in FIG. 2, until the horn 5 is attained, thereby establishing a connection bridge, as indicated by the-reference numeral 13 in the FIG. 12, between the contact 14 and born 5. Then, due again to the magnetic action by which the arc is urged upwardly, an extension of the above-considered arc root will leave the contact 14, to climb the horn 5, thus taking positions like that indicated, by way of example, by the reference numeral 13 in FIG. 12, whereafter the arc passes under the reach of the magnetic field of the magnet pair 18, and by which it is compelled to ascend toward the upper end of horn 5.

Thus, even in the latter case, the most part of the electric arc is quickly removed from the contact gap, whereby the air, or the other fluid present between the contacts, is enabled to recover its insulating properties as quickly as possible. In the meantime, the resistor, by which the current to be cut off is limited, has been gradually inserted into the related circuit, thereby reducing current to values such that final extinction of arc can be readily attained.

The omission of connections, as previously provided for the contacts, represents a remarkable improvement and simplification of the extinction chamber with which the invention is concerned.

FIG. 11 diagrammatically shows the main components of a breaking device, that operates according to inventive idea, the design of the components such as to allow the utilization of the device inside of a cylindric container.

The design of all heretofore described extinction chambers is such as to allow rectilinear motion only of movable co'ntact. Conversely, the extinction chamber 203 of device shown in the FIG. 11, is designated in such a manner as to allow the movable contact 205 to travel along a curved path, coincident with the curvilinear axis of contacts that are fitted within the chamber.

The chamber is supported by an insulating support 204, while the movable contact is supported and operated by a support 206, alsomade of an insulating material.

Also the whole breaking device may be enclosed within the container 2|",should an insulating medium, other than the air, be utilized in the device.

While specific embodiments of the invention have been I shown and described in detail to illustrate the application of the principles of the'invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

lclaim:

1. In a device for quenching the arc struck between current carrying contacts of an electric circuit breaker when the contacts are separated under load, of the type including a quenching chamber, delimiting the path and extension of the arc, and ot' the type utilizing magnetic field means to draw the are from the contact gap and to extend and quench the arc: the improvement comprising, in combination, at least one relatively elongated resistor positioned outside said quenching chamber with one end immediately adjacent the arcing and contactgap defining end of one of said contacts for current flow from said one contact to said one end of said resistor; said resistor extending from said one contact in thedirection in which the arc is drawn from the contact gap by said magnetic field means; whereby, when said contacts are separated under load, one end of the resulting arc will travel from said one contact outwardly along and in contact with said resistor, as

' the arc is drawn from the contact gap by said magnetic field means, to insert a progressively increasing length of said resistor in the circuit including said contacts to decrease the current flow in the circuit.

2. In a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim 1, in which said one end of said resistor is electrically connected to said one contact.

, 3. In a device for quenching the arc struck between current g contacts, the improvement claimed in claim 2, in which there is a single ohmic resistor adjacent one end of said quenching chamber; said one contact comprising a fixed contact and being cooperable with a movable contact to define the contact gap; and a rear horn of electrically conducted material at the opposite end of said quenching chamber; and flexible and sliding connections electrically connecting said rear hom to said movable contact.

4. In a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim 2, in which said one contact is cooperable with a second contact to define the contact gap, one of said first and second contacts being movable relative to the other thereof to establish the contact gap; there being two said relatively elongated resistors, which are ohmic resistors, positioned outside and adjacent respective opposite ends of said quenching chamber and each having one end in electrically conductive connection with a respective one of said contacts; each resistor extending from its associated contact in the direction in which the arc is drawn from the contact gap by said magnetic field means; whereby, when said first and second contacts are separated under load, opposite ends of the resulting arc will travel from the respective contacts upwardly along and in contact with the associated resistors, to insert progressively increasing lengths of said resistors in the circuit including said contacts to decrease the current flow in the circuit.

5. in a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim I, in which said magnetic field means comprises pairs of permanent magnets with the permanent magnets of each pair being interconnected by paramagnetic return circuits; said pairs of permanent magnets being positioned both outside :of and inside said quenching chamber and being disposed all along the arc path and extension of the are as delimited by, said quenching chamber.

6. In a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim I, in which said magnetic field means includes an electromagnet havingan excitation winding energized by the same current that flows through the circuit including said contacts.

7. In a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim 1, in which said quenching chamber consists of two identical sections located symmetrically opposite each other with respect to a plane passing through the centerline of movement of a movable contact included in said current carrying contacts; each section acting on the electric arc produced by a respective relative polarity of the current flow between said contacts.

8. In a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim 1, in which said current carrying contacts include a fixed contact, a first movable contact, and a second movable contact 'intennediate said fixed contact and said first movable contact;

said quenching chamber being subdivided into first and second consecutively positioned secondary chambers operated in sequence by the progressive opening movement of said intermediate movable contact; whereby a first arc length is struck in the first chamber following which a second arc length is struck in the second chamber so that the most intensive ionizing action, as caused by the arc, is restricted to said first chamber wherein intensive arc flow current will flow at the initiation of contact separation; the current intensity of such second arc length in said second chamber being decreased by virtue of insertion of a progressively increasing length of said resistor into the circuit including said contacts by said first arc length.

9. In a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim 1, in which each resistor comprises ceramic bodies and electrically conductive plates interposed between adjacent ceramic bodies; and clamping means pressing said ceramic bodies into firm contact with said electrically conductive plates.

10. In a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim 1, in which at least one of said resistors is a nonlinear resistor. 

1. In a device for quenching the arc struck between current carrying contacts of an electric circuit breaker when the contacts are separated under load, of the type including a quenching chamber, delimiting the path and extension of the arc, and of the type utilizing magnetic field means to draw the arc from the contact gap and to extend and quench the arc: the improvement comprising, in combination, at least one relatively elongated resistor positioned outside said quenching chamber with one end immediately adjacent the arcing and contact gap defining end of one of said contacts for current flow from said one contact to said one end of said resistor; said resistor extending from said one contact in the direction in which the arc is drawn from the contact gap by said magnetic field means; whereby, when said contacts are separated under load, one end of the resulting arc will travel from said one contact outwardly along and in contact with said resistor, as the arc is drawn from the contact gap by said magnetic field means, to insert a progressively increasing length of said resistor in the circuit including said contacts to decrease the current flow in the circuit.
 2. In a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim 1, in which said one end of said resistor is electrically connected to said one contact.
 3. In a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim 2, in which there is a single ohmic resistor adjacent one end of said quenching chamber; said one contact comprising a fixed contact and being cooperable with a movable contact to define the contact gap; and a rear horn of electrically conducted material at the opposite end of said quenching chamber; and flexible and sliding connections electrically connecting said rear horn to said movable contact.
 4. In a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim 2, in which said one contact is cooperable with a second contact to define the contact gap, one of said first and second contacts being movable relative to the other thereof to establish the contact gap; there being two said relatively elongated resistors, which are ohmic resistors, positioned outside and adjacent respective opposite ends of said quenching chamber and each having one end in electrically conductive connection with a respective one of said contacts; each resistor extending from its associated contact in the direction in which the arc is drawn from the contact gap by said magnetic field means; whereby, when said first and second contacts are separated under load, opposite ends of the resulting arc will travel from the respective contacts upwardly along and in contact with the associated resistors, to insert progressively increasing lengths of said resistors in the circuit including said contacts to decrease the current flow in the circuit.
 5. In a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim 1, in which said magnetic field means comprises pairs of permanent magnets with the permanent magnets of each pair being interconnected by paramagnetic return circuits; said pairs of permanent magnets being positioned both outside of and inside said quenching chamber and being disposed all along the arc path and extension of the arc as delimited by said quenching chamber.
 6. In a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim 1, in which said magnetic field means includes an electromagnet having an excitation winding energized by the same current that flows through the circuit including said contacts.
 7. In a device for quenchiNg the arc struck between current carrying contacts, the improvement claimed in claim 1, in which said quenching chamber consists of two identical sections located symmetrically opposite each other with respect to a plane passing through the centerline of movement of a movable contact included in said current carrying contacts; each section acting on the electric arc produced by a respective relative polarity of the current flow between said contacts.
 8. In a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim 1, in which said current carrying contacts include a fixed contact, a first movable contact, and a second movable contact intermediate said fixed contact and said first movable contact; said quenching chamber being subdivided into first and second consecutively positioned secondary chambers operated in sequence by the progressive opening movement of said intermediate movable contact; whereby a first arc length is struck in the first chamber following which a second arc length is struck in the second chamber so that the most intensive ionizing action, as caused by the arc, is restricted to said first chamber wherein intensive arc flow current will flow at the initiation of contact separation; the current intensity of such second arc length in said second chamber being decreased by virtue of insertion of a progressively increasing length of said resistor into the circuit including said contacts by said first arc length.
 9. In a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim 1, in which each resistor comprises ceramic bodies and electrically conductive plates interposed between adjacent ceramic bodies; and clamping means pressing said ceramic bodies into firm contact with said electrically conductive plates.
 10. In a device for quenching the arc struck between current carrying contacts, the improvement claimed in claim 1, in which at least one of said resistors is a nonlinear resistor. 